1. Field of the Invention
The present invention relates to a disk array device, a parity data generating circuit for RAID (Redundant Array of Independent Disks) and a Galois field multiplying circuit, and particularly, relates to a disk array device of RAID6, a parity data generating circuit for RAID and a Galois field multiplying circuit requiring high reliability and high performance.
2. Description of the Related Art
In the disk array device for storing important data, reliability is increased by a RAID technique or the like conventionally known, and plural RAID systems of RAID0 to RAID5 have been defined. However, in recent years, a system of RAID6 improving these plural RAID systems has appeared and a further improvement of reliability is required.
The reliability of hardware of the disk array device itself is also required. In data transfer, a mechanism for adding a check code of data guarantee such as parity and ECC (Error Correcting Code) and preventing the transfer data from being changed into another code has been indispensable.
In particular, in the RAID6 recently defined, it is necessary to multiply the Galois field upon treating data. With respect to its arithmetic results, a new mechanism for improving reliability such as reflection of parity for securing the justifying property of data is required.
For example, a RAID device for realizing the RAID6 by using an extension Galois field GF(24) capable of treating 16 kinds of data in the calculation of parity is known (see Japanese Unexamined Patent Publication (JP-A) No. 2000-259359). Hereinafter, the extension Galois field is suitably simply called Galois field.
FIG. 1 shows a circuit construction for generating parity data for RAID in the prior art. In transfer data 20-1 from a memory, the circuit is constructed such that data 20-2, parity 20-3 and a multiplication coefficient 204 of the Galois field with respect to these data 20-2 are transferred. With respect to the data 20-2 themselves, a parity check is made by a parity check circuit 20-5 (see “A Tutorial on Reed-Solomon Coding for Fault-Tolerance in RAID-like System” James S. Plank, Department of Computer Science University of Tennessee, Feb. 19, 1999)
An arithmetic calculation using a Galois field multiplying circuit 20-6 is made by designating the multiplication coefficient 20-4 for these data 20-2 so that data 20-7 for RAID are generated. After this generation, the parity for the data 20-7 is generated by a parity generating circuit 20-8. Thus, the justifying property of the data 20-2 from the memory is guaranteed by the parity check circuit 20-5.
In a data guaranteeing circuit in the conventional method as shown in FIG. 1, there is an effect in a logic failure or the like. However, no failure of hardware can be found when a breaking point is narrowed down to a certain one point as in deterioration due to a change with the passage of time within LSI (Large Scaled Integration) and a short circuit.
This problem will be explained in the circuit construction of FIG. 1. Specifically, a state guaranteeing no data 20-2 is attained because no parity error can be detected even if the parity error is generated when the parity check circuit 20-5 for guaranteeing data is broken.
Further, data having no problem are used upon checking the error. However, when the Galois field multiplying circuit 20-6 is broken, its arithmetic result is converted into data except for an object and parity 20-12 is simultaneously newly given again by the parity generating circuit 20-8 existing after this conversion. No parity error is detected even when the data 20-7 are converted in error. The parity data Q for RAID finally generated cause a data changing state, which becomes a fatal wound as a disk array system.
In order to avoid such a problem, proposal has been made of a conventional technique for doubling the Galois field multiplying circuit 20-6 itself and comparing the mutual arithmetic results. However, when an error signal itself showing the comparing result is broken by a short circuit, no effect can be shown at all.
In the disk array device of RAID6 requiring high reliability, the event that these abnormal data are considered as effective data should not be caused.